The air-liquid interface in the lung is a region of high surface tension that, if unmodified, causes alveolar collapse and respiratory failure. Under normal conditions, a surfactant film consisting of saturated phospholipids that is synthesized and secreted by alveolar type II cells reduces this high surface tension. Pulmonary surfactant also contains a family of proteins, SP-A, SP-B, SP-C, and SP-D, which variously contribute to formation of the surfactant film, surfactant homeostasis, and host defense. Pulmonary surfactant deficiency or alteration has been incontrovertibly linked to respiratory distress syndrome (RDS) in neonates and contributes to the lung dysfunction seen in acute respiratory distress syndrome (ARDS) in the mature lung. Phospholipids constitute approximately 85% of the mass of pulmonary surfactant. The single most abundant phospholipid, is saturated phosphatidylcholine (SatPC), particularly dipalmitoylphosphatidylcholine (DPPC). SatPC can be synthesized either de novo or by a mechanism involving the deacylation of monosaturated PC followed by reacylation, usually with palmitate. Studies on surfactant synthesis in vitro have shown that the latter pathway accounts for 55- 75% of the DPPC made by type II cells. Although activity of a lysoPC acyltransferase utilized in reacylation was demonstrated in type II cells over 30 years ago, the corresponding gene has not been heretofore identified. Data accompanying this application demonstrate that we have identified and isolated the mouse cDNA for a lysoPC acyltransferase (LPCAT1) that is highly enriched in type II cells. Using a hypomorphic transgenic mouse, we also demonstrated that decreased LPCAT1 expression reduces SatPC levels in the developing lung and directly correlates with survival. We will extend these studies in three Specific Aims. In the first Specific Aim we will completely inactivate the LPCAT1 gene in the fetus and determine how this affects survival, phospholipid synthesis, and overall gene expression in the lung. In Specific Aim 2 we will generate a conditional knockout of LPCAT1 that will allow us to define the relationship between SatPC pool size and lung function in the adult animal. In Specific Aim 3 we will exploit the role of LPCAT1 in SatPC synthesis to determine its role in the initiation of SatPC transport to lamellar bodies. The completion of these studies will fill important gaps in our knowledge regarding surfactant phospholipid synthesis, and may significantly impact strategies for the treatment of RDS and other lung diseases resulting from an insufficiency in pulmonary surfactant.